1. Field of the Invention
The present invention relates to an aluminum nitride substrate for use as a substrate for depositing a thin film circuit or the like, and a thin film substrate therewith and a manufacturing method thereof.
2. Description of the Related Art
In a microwave integrated circuit, since a semiconductor chip may be directly connected to an impedance matching circuit in a microwave band, an influence of parasitic reactance due to a package or a lead wire may be remarkably diminished. As a result, a small and accurate microwave frequency band active circuit may be realized. Recently, by making use of such advantages, the microwave integrated circuits have began rapidly expanding uses as hybrid ICs for optical communication, hybrid ICs for mobile communication, hybrid ICs for laser diodes, hybrid ICs for automobiles and so on.
In the microwave integrated circuit, since higher accuracy and higher reliability are demanded, thin film deposition technique, such as sputtering, vacuum vapor deposition, and CVD, in particular, PVD (Physical Vapor Deposition), is generally used for circuit formation. In the circuit (thin film circuit), to which the thin film deposition, such as the PVD or the like, is applied, patterning accuracy is superior by one digit or more to that of a thick film circuit; film material is higher in purity; and, furthermore, accuracy, noise characteristics, temperature characteristics and stability of a film element are excellent.
In a thin film hybrid IC in which a thin film circuit is applied, since the circuit may be highly integrated, heat generation amount accompanying circuit operation is in an increasing tendency. Furthermore, since the development of a semiconductor chip of higher power is in progress, the heat generation amount from the semiconductor chip is increasing year by year. Accordingly, the substrate for the microwave ICs is important to be excellent in heat dissipation characteristics, and aluminum nitride substrates, which are excellent in thermal conductivity, are in heavy usage. Thus, for the microwave IC substrates, thin film substrates, in which a thin film circuit is deposited on the aluminum nitride substrate that is excellent in thermal conductivity, are in heavy usage. The thin film substrates are also used for substrates for sub-mounting the laser diodes.
Furthermore, for the substrate for microwave ICs, in addition to excellent heat dissipation characteristics, surface properties capable of depositing a thin film circuit with accuracy are demanded. That is, when the thin film circuit is deposited by means of the sputtering or the like, the surface properties of a thin film surface (circuit surface) are important; when there is an unevenness on the surface, the circuit is deposited with deteriorated accuracy. Accordingly, as disclosed in, for instance, Japanese Patent Laid-Open Application No. 11-31869 JP-A, in case an aluminum nitride sintered body or the like is used as a thin film substrate, mirror finish is generally applied on the surface thereof. In the aforementioned document, the surface roughness Ra of an insulating substrate, such as the aluminum nitride substrate or the like, is set at 0.1 xcexcm or less.
In particular, in the aluminum nitride sintered body, when the heat dissipation properties are improved, there is a tendency in that a sintering additive, which forms a liquid phase that may deteriorate the thermal conductivity, is added less. In case the sintering additive is added less, when a sintering temperature is not raised high enough, sintering characteristics deteriorate. However, when the sintering temperature is raised, the sintering additive component (grain boundary phase component) precipitates on a surface of a sintered body, and at the same time, grain growth of aluminum nitride crystal grains tends to occur. The sintering additive component present on the substrate surface lowers a joining strength of the thin film. Furthermore, when there are excessively grown aluminum nitride crystal grains on the substrate surface, since flatness on the substrate surface is deteriorated, it becomes difficult to uniformly deposit a thin film. Accordingly, when a thin film is deposited on the surface of the aluminum nitride sintered body (substrate), the thin film surface is generally mirror-finished. Furthermore, an acid wash is effectively performed after the mirror finish, thereby removing contamination on the substrate surface.
In the existing aluminum nitride sintered body, during the mirror polish or acid wash, the grain boundary phase component present on the surface of the sintered body is likely to fall off or be dissolved. Accordingly, there is a problem in that the thin film is difficult to deposit with high accuracy. As disclosed in the aforementioned Japanese Patent Laid-Open Application No. 11-31869 JP-A, simple mirror polish of the surface of the existing aluminum nitride sintered body is likely to cause the fall off of the grain boundary phase component, which precipitates on the surface thereof, and the aluminum nitride crystal grain. Furthermore, the dissolution or the like of the grain boundary phase component is also likely to be caused at the acid wash. These may cause a relatively large pit. When a thin film is deposited on the surface of the substrate having such a pit, a space may be caused between the thin film and the substrate. The space between the thin film and the aluminum nitride substrate may cause swelling due to heat applied in the later course of manufacture and during the use of the circuit, resulting in lowering the circuit accuracy or in peeling the thin film.
As to the surface properties of the aluminum nitride substrate, for instance, Japanese Patent Laid-Open Application No. 2000-281427 JP-A discloses a substrate in which skewness of a component-mounting surface or a circuit surface is set at 0 or less. In this document, in order to strengthen the joining strength with a metal plate, such as a copper plate, the skewness of the substrate surface is controlled based on the polishing conditions or the like. However, simple control of the polishing conditions of the aluminum nitride sintered body may not necessarily sufficiently satisfy the characteristics demanded for the thin film surface.
Furthermore, Japanese Patent Laid-Open Application No. 5-238830 JP-A discloses an aluminum nitride sintered body, in which, in order to improve the thermal conductivity and the mechanical strength of the aluminum nitride sintered body, the maximum diameter of the grain boundary phase in an internal crystal texture is set at 1 xcexcm or less, and furthermore the maximum diameter of pore is set at 1 xcexcm or less. Reduction of the grain boundary phase and the pore inside of the sintered body may contribute in an improvement of the thermal conductivity. However, as mentioned above, since, in the thin film substrate, the surface state may cause problems, simple control of an amount of the grain boundary phase inside the sintered body may not sufficiently satisfy the characteristics demanded for the thin film surface. In particular, in the aluminum nitride sintered body, in which high thermal conductivity is intended, since the grain boundary phase tends to precipitate on the sintered body surface, simple control of the amount of the grain boundary phase inside of the sintered body may adversely affect on the surface characteristics.
Accordingly, an object of the present invention is to provide an aluminum nitride substrate that, in addition to making use of high heat dissipation characteristics of the aluminum nitride substrate, allows improving intimate contact properties with a metal thin film, which is used for depositing various kinds of circuits, and accuracy in thin film deposition. Another object of the present invention is to provide a thin film substrate, in which, by use of such aluminum nitride substrate, reliability or operation characteristics are improved, and a manufacturing method thereof.
The aluminum nitride substrate of the present invention, one that contains a rare earth oxide as a sintering additive component, includes a surface that is machined so that arithmetic average roughness Ra may be 0.5 xcexcm or less, and an aggregate size of the sintering additive component, which is present on the machined surface, is controlled to 20 xcexcm or less, and a total aggregate area in a unit area of the machined surface is controlled to be 5% or less.
In the aluminum nitride substrate of the present invention, the aggregate size of the sintering additive component, which is present on a substrate surface after the machining (machined surface), and the total aggregate area are controlled in predetermined ranges. When the aluminum nitride sintered body is used for the thin film substrate, mirror finish of the surface (thin film surface) thereof is necessary and a further acid wash is effective.
Accordingly, by reducing the size and the total aggregate area of the sintering additive component, which precipitates on the surface of the aluminum nitride sintered body, the pit (cave) due to fall off or dissolution of the aggregate of the sintering additive component in the course of mirror finish or acid wash may be suppressed from occurring. In particular, the pit that is generated due to the dissolution of the aggregate of the sintering additive component during the acid wash may be largely suppressed from occurring. Thereby, when the metal thin film is deposited on the surface of the aluminum nitride substrate by means of PVD method, such as sputtering or vapor deposition, the intimate contact properties of the metal thin film and the deposition accuracy thereof may be largely increased.
The thin film substrate of the present invention includes the aforementioned aluminum nitride substrate of the present invention, and the metal thin film deposited on the machined surface of the aluminum nitride substrate. The thin film substrate of the present invention may be suitably applied to substrates for microwave integrated circuits and sub-mount substrates on which a laser diode is mounted.